Zirconium oxide solid-electrolyte fuel cells are electrochemical energy converters which generate current directly from gaseous energy carriers (such as H.sub.2, CO, CH.sub.4). They are based on zirconium oxide as an oxygen ion conducting solid electrolyte and are operated at temperatures of approximately 800.degree. C. to 1,000.degree. C. Since they are not subject to the Carnot Rule like thermal engines, they reach clearly higher efficiencies of about 50%. For this reason and because of their lower emission of pollutants, they have great potential as future energy converters, particularly if they use natural gas as the primary energy carrier.
In such fuel cells, a planar cell design is advantageous with respect to efficiency and high energy density. In this type of arrangement, thin solid-electrolyte plates are coated on both sides with porous electrodes and the coated plates are stacked above one another alternately with joining elements so that a bipolar arrangement is created. Several individual cells may be connected in series. These cell stacks are connected to form larger units by means of electric conductor systems and gas distribution lines.
The arrangements described above for fuel cells may also be used for the reverse process of high-temperature electrolysis in order to produce hydrogen from water at high efficiency at temperatures of approximately 800.degree. C. to 1,000.degree. C.
If, as currently preferred in the art, cross-flow gas lines are used, then it is considered advantageous to use gas distribution and collection components made of zirconium oxide or aluminum oxide. If other types of gas lines, such as internal gas lines are used, however, then metallic parts may also be used in part. However, such metallic parts must be electrically insulated with respect to the cells. However, all these solutions are subject to fundamental disadvantages. Zirconium oxide has the disadvantage that it is conductive to oxygen ions so that an electric shunt exists to the cells which lowers the efficiency and may even result in electrochemically induced degradation of materials used to construct the cell. If aluminum oxide is used, an expensive construction is required in order to compensate for the unequal thermal expansion between the cell stack and the gas supply components. Frequently, however, such compensating constructions frequently can only be achieved at the expense of gas-tightness. If metallic parts are used, complicated geometries are required for electrically insulating the cells, which cause high costs and adversely affect the manufacturing and operational reliability.
Materials used to construct solid-electrolyte fuel cells preferably include the following:
Electrolyte: ZrO.sub.2 doped with CaO, MgO, Y.sub.2 O.sub.3, or another rare-earth oxide, and optionally also containing added Al.sub.2 O.sub.3 ; PA1 Fuel Gas Electrode: Metal/ceramic composite materials containing nickel or cobalt as a metallic component and doped CeO.sub.2 or ZrO.sub.2 as the ceramic component; PA1 Air Electrode: Doped oxide having a perovskite structure, such as La.sub.1-x Ca.sub.x MnO.sub.3, La.sub.1-x Sr.sub.x MnO.sub.3, La.sub.1-x Sr.sub.x Co.sub.y Mn.sub.1-y O.sub.3 ; PA1 Connecting Element: Doped lanthanum chromite, such as La.sub.1-x Sr.sub.x Co.sub.y LaMg.sub.x Cr.sub.1-x O.sub.3. PA1 The thermal expansion of the gas distribution and collection components must correspond to that of the other fuel cell components, particularly the ZrO.sub.2 -electrolyte. PA1 The gas distribution and collection components must be resistant to high temperatures in the presence of air, on the one hand, as well as to fuel gases, such as H.sub.2, CH.sub.4 or CO, on the other hand. PA1 The gas supply components must be electrically insulating at their operating temperature so that they do not represent a shunt to the cell stack. PA1 The gas supply components must be capable of being manufactured at reasonable cost, and it must be readily possible to connect them to the cell stack in a gas-tight manner.
The foregoing components are united in the form of cell stacks to form gas-tight units.
In order to meet strict requirements with respect to gas-tightness at high temperatures as well as during heating and cooling, the coefficients of thermal expansion of all components must be well matched to each other.
Air and fuel gas are supplied under defined flow conditions to the cell stacks, and the waste gases or the depleted air are removed. For this purpose, gas-tight gas distribution and collection boxes are needed which must meet the following requirements:
Prior to the present invention, no unitary system was known that could meet all four of the foregoing requirements.